This invention relates to improvements in fault protection apparatus for protecting electrical power lines, and in particular to an improved time graded fault protection apparatus.
One of the most important fault protection techniques for power lines is known as Overcurrent Protection. In this technique, a circuit breaker is provided which in normal operation is closed to connect two portions of a line. A current measurement means, typically a current transformer, monitors current flowing in the line through the circuit breaker. This is often referred to as a relay. The output of the current measurement means is monitored by a fault detection means, and in the event that the output corresponds to a current indicative of a fault condition the circuit breaker is opened to isolate one portion of the line from the other.
On extended lengths of power line, more than one circuit breaker (and associated current measuring means and fault detection means) are provided. These are spaced apart along the electrical line. In this case, to ensure that only the minimum number of circuit breakers are opened to clear a fault, a time grading strategy is employed whereby each circuit breaker is adapted to be opened after a predetermined time has elapsed following detection of a fault by its associated fault detection means. The current detection means are made directional, i.e. to monitor the current flowing in each direction, and the time delays for each circuit breaker are graded according to their position relative to other circuit breakers on the line, and the direction along the line from the circuit breaker in which the fault has been detected.
A typical prior art apparatus involving five circuit breakers is shown in FIG. 1. The circuit breakers are denoted CB1, CB2, CB3, CB4 and CB5 and are provided in series between a first end xe2x80x98Rxe2x80x99 of a protected line and a second end xe2x80x98Rxe2x80x99. Each circuit breaker is provided with two current measurement relays, one for each direction. The time delays for each circuit breaker, starting with that nearest the first end are T1, T2, T3, T4 and T5 were T5 greater than T4 greater than T3 greater than T2 greater than T1.
These time delays relate to the time which elapses after a fault has been detected at a respective circuit breaker until the breaker is opened for a fault in the direction of the first end of the line, i.e. to the left of the circuit breaker.
For a fault in the direction of the second end of the line, the delays are (starting with the circuit breaker nearest the second end of the line) T1xe2x80x2, T2xe2x80x2, T3xe2x80x2, T4xe2x80x2 and T5xe2x80x2 where T5xe2x80x2 greater than T4xe2x80x2 greater than T3xe2x80x2 greater than T2xe2x80x2 greater than T1xe2x80x2. Each circuit breaker therefore has two time delays associated with the breaker. With the exception of the central breaker CB3, these will be different valued.
As can be seen from FIG. 1, the circuit breakers nearest an end of the line open after a shorter delay for current flowing from the direction of the near end.
A problem with the prior art time grading technique arises where line lengths are long and a high number of circuit breakers are provided. In such a case, the some of the time delay may be unacceptably high. Also, the time delays are fixed in value once the apparatus is installed.
In accordance with a first aspect of our invention in a fault protection apparatus for use in a time graded fault protection scheme comprising a circuit breaker means having a normally closed trip contact provided in a line, current measurement means adapted to produce an output signal indicative of the current in the line at the circuit breaker means, and fault signal processing means adapted to process the output signal from the current measurement means to detect an initial fault on the line at a first instance, the fault signal processing means is further adapted to process the output of the current measurement means after the initial fault has been detected and to produce an accelerated opening signal in the event that the output signal meets a predetermined criteria, and control means are adapted to open the circuit breaker trip contact in response to said accelerated opening signal.
The output signal may be adapted to meet the predetermined criteria when an event on the line indicative of the operation of a remote circuit breaker means on the line occurs and the output signal after the operation remains indicative of a fault on the line. Under these conditions on accelerated opening signal may be produced and the circuit breaker may then be opened instantly. If the output signal indicates that a remote circuit breaker has opened and the fault has been cleared from the protected portion of line, no accelerated trip signal will be generated and the circuit breaker may be kept closed.
The current measurement means may comprise a forward and reverse current measurement means adapted to produce an output signal indicative of the current flowing in the line on either side of the fault protection apparatus. This allows the apparatus to decide where the fault is on the line. Two fault signal processing means may then be provided, one for each direction. A directional response to faults can then be achieved.
The fault signal processing means may be adapted to produce an accelerated opening signal if a fault condition meeting the predetermined criteria is detected by the forward current measurement means but not by the reverse opening means. If a fault is initially detected by the reverse current measurement means, a time delayed opening signal may be generated which is used by the control means to open the circuit breaker after a predetermined time delay, say 0.5 seconds.
In accordance with a second aspect of our invention in a directional time graded fault protection apparatus for an electrical line comprising at least two fault protection apparatus respectively provided at a first and second end of a protected portion of the line, each fault protection apparatus comprising at least one circuit breaker means, a forward and reverse current measurement means associated with each circuit breaker means, the forward current measurement means being adapted to measure current flowing in the line from a first direction and, the reverse current measurement means being adapted to measure current flowing in the line from the second opposite direction, control means associated with each circuit breaker, each control means being adapted to open its respective circuit breaker after a first predetermined period of time in response to an initial fault detected by processing the output of the forward current measurement means associated with the breaker and fault signal processing means associated with at least one of said circuit breakers adapted to monitor the output of the reverse current measurement means so that in the event that the output from the reverse current measurement means meets a predetermined criteria after an initial fault has been detected, the control means is adapted to open the respective circuit breaker at an accelerated time.
If the output signal meets the predetermined criteria, the fault signal processing means may be adapted to produce an accelerated opening signal which is used by the control means to open the circuit breaker.
If the output signal does not meet the predetermined criteria, the control means may cause the circuit breaker to open after a predetermined time period has elapsed since an initial fault is detected by the reverse current measurement means.
Thus, the invention provides a time graded fault protection apparatus for a line in which at least one fault protection apparatus is adapted to make an accelerated opening decision for certain fault conditions which increases speed of response. In prior art systems, the slower reacting fault protection apparatus (those at a far end from a fault) can only open the circuit breaker after their predetermined time delay has elapsed following initial fault detection.
The current measurement means may, in an alternative, be replaced by a voltage signal measurement means adapted to measure the voltage on the line. This may be processed to produce a current signal.
There may be additional fault protection apparatus comprising one or more circuit breakers, associated control means and current measurement means provided in the line. Not all of the additional circuit breakers need to be able to open at an accelerated rate. For example, these additional fault protection apparatus may be adapted to monitor the output of the forward and reverse current measurement means and to open the circuit breaker after a first predetermined time delay when the output of the forward circuit measurement means indicates an initial fault on the line, and after a second time delay for faults detected by monitoring the output of the reverse current measurement means.
Preferably, the second time period which elapses is greater than the first time period. Those circuit breakers which are further away from the end of the line from which the current flows may have increasing time delays for that direction.
A fault signal processing means may be associated with each respective circuit breaker. This may be adapted to produce an output signal indicative of an initial fault by processing the output from the current measurement means.
The predetermined criteria used by the fault signal processing means to make the accelerated opening decision may be satisfied if the current measured after the initial detection of a fault indicates that a remote circuit breaker on the line in the direction of the measured fault has opened and the fault has not been cleared from the protected portion of line.
The fault signal processing means may therefore be adapted to detect the operation of another circuit breaker on a portion of line in the direction of the detected fault by processing the output of the current measurement means.
Where the power line is a three-phase line, the current measurement means may be adapted to produce three output sub-signals. Each signal may be associated with a respective phase of the three phase supply. The output sub-signals may be continuous or discontinuous, i.e. a sequence of measured values over time. The output signals may be indicative of the current and/or voltage on the line. They may be digital or analogue.
The fault signal processing means may be adapted to produce an accelerated trip signal by combining values of the output signals obtained substantially at the instant that the initial fault is detected with values of the output signals obtained after the initial fault is detected.
The circuit breaker means may comprise a forward circuit breaker and a reverse circuit breaker associated with the forward and reverse current measurement means respectively. Alternatively, it may comprise a single circuit breaker. The forward and reverse circuit breaker may be located on either side of a load on the line to protect the load. This enables a load to be isolated from a fault regardless of which side of the line it occurs.
Most preferably, the fault signal processing means is adapted to detect changes in the output signals over time after an initial fault in order to sense the operation of another circuit breaker on the line in the direction of the fault.
The fault signal processing means may be adapted to produce an initial fault signal in the event that the output signals indicate an unbalanced fault current condition on the line.
The three-phase system may be represented as the sum of three phase components; a positive sequence having the same phase rotation as the original system, a negative sequence having the opposite phase rotation, and a zero sequence having no phase rotation.
A system is said to be unbalanced when the three currents and/or voltages signals are not of equal amplitude and/or the phase differences between successive signals are unequal.
An unbalanced fault current condition may therefore be detected by providing means adapted to monitor the negative and zero sequence quantities in the output signals.
Other faults which are symmetrical (such as three phase to earth faults) may be detected by monitoring changes in the positive sequence quantity. Changes in the zero and/or positive and/or negative sequence quantities after the initial fault is detected can be used to detect operation of the remote circuit breaker.
A combination of the negative, zero and/or positive sequence values before and after opening of the second circuit breaker is detected can be used to determine the location of a fault of the line. For instance, this can be used to decide if the fault is on the portion of line between the first and second circuit breakers.
The fault signal processing means may be adapted to calculate the RMS (root mean square) values of one or more of the sequence quantities in a first step of processing the sequences to produce the initial fault signal.
The processing means may calculate one or more ratio signals from the measured sequence values. The value of the ratio signals can be used as an indication of a fault condition. For example, if a ratio exceeds a predetermined threshold, a fault can be assumed to have occurred.
A first ratio may comprise the change in the zero and negative sequence quantities. This ratio may, for example, change either on transition from non-fault to a fault condition or upon closure of a remote circuit breaker.
A second ratio may comprise the change in the positive sequence quantity during the time period following initial fault detection. This can provide an indication of the presence of the symmetrical fault.
The first ratio signal may be chosen so as to have a zero value under no fault or symmetrical fault condition as there will be no negative or zero sequence quantities. The first ratio will increase from zero if a unsymmetrical fault is detected. This increase can trigger the initial fault signal. If, subsequently, the ratio returns to zero, the fault has either cleared (transistent fault) or a remote breaker has opened to isolate the fault.
If a symmetrical fault occurs, the second ratio will increase from zero or will return to zero if the fault is cleared by the remote breaker.
Of course, the output signals can be processed in either ways to indicate the presence of a fault and the operation of a remote breaker.